Insulated conductor having adhesive overcoat

ABSTRACT

Disclosed is a conductor coated with a fused and cured powder applied insulating coating over which is a coating of an ultraviolet B-stageable, thermally C-stageable liquid resin. The coating is cured to the B-stage and strands of the conductor are placed side-by-side. The coating is then cured to the C-stage which fuses the strands of the conductor into a solid mass. The adhesive coated conductors may be used to form transformer coils, transposed cables, or other articles.

BACKGROUND OF THE INVENTION

Electrical failure of transformer coils, transposed cables, and otherelectrical equipment can occur when short circuit forces or mechanicalabuse damage insulation. The mechanical strength of insulated coils andcables can be increased by bonding the individual insulated electricalconductors together into a single mass. This has been accomplished byplacing adhesive coated paper in between the layers of conductors.However, while this increases strength and reduces insulation damage, italso increases the cellulosic content of the electrical apparatus.Cellulose is undesirable if certain dielectric fluids are presentbecause they react with cellulose to produce compounds which increasethe conductivity of the fluid. Another technique for forming a singlemass out of multiple conductors is to cover the conductors with anadhesive either before or after the conductors have been juxtaposed.While this technique has also worked, it involves an additional step,and difficulties may be encountered in obtaining good adhesion betweenthe insulation and the adhesive.

SUMMARY OF THE INVENTION

We have discovered a method of preparing an adhesively coated insulatedconductor which can be bonded to itself to form a solid mass which isresistant to electrical stress and mechanical abuse. The adhesivelycoated conductor according to this invention can be made in a singlepass in a manufacturing process that requires very little space. Theadhesive overcoat can be rapidly cured to the B-stage with ultravioletlight (UV) which requires less energy than a thermal cure. It can thenbe easily thermally cured to C-stage (i.e., completely cured) once theconductors have been formed into a coil or cable. Since the overcoat is100% solids no solvent is evolved during curing and thus problems of airpollution and the collection and containment of vaporized solvent areavoided. The adhesive coated conductor has an excellent shelf life andcan be stored for long periods of time prior to use. The overcoat doesnot flake and adds to the insulating qualities of the undercoat.

While ultraviolet curable compositions are not meant to be cured byheat, we have found that such compositions can be very usefully adaptedto producing adhesive coatings by only partially curing them withultraviolet light and later completing the cure with heat. In spite ofthis unusual use of UV compositions, we have obtained excellentadherence between bonded conductors, and no adverse reactions have beenobserved that have lowered electrical or mechanical properties.

Surprisingly, we have discovered that the overcoat has a synergisticinteraction with a powder coated undercoat in that the dielectricstrength of the overcoat on top of the undercoat is greater than the sumof the dielectric strengths of the two coatings by themselves.

RELEVANT ART

U.S. Pat. Nos. 3,619,259 and 3,911,202 disclose UV polymerization ofcontinuous films which may be used for the purpose of electricalinsulation.

U.S. Pat. Nos. 4,184,001 and 4,268,659 disclose UV curable compositionsspecifically for use as insulation of electric wires.

U.S. Pat. No. 4,317,858 discloses a UV curable adhesive.

U.S. Pat. Nos. 4,032,673 and 4,239,077 disclose UV curable resins foruse in transformer coils.

DESCRIPTION OF THE INVENTION

The accompanying drawing is an isometric view in section of a certainpresently preferred embodiment of an insulated conductor having anadhesive overcoat according to this invention.

In the drawing, conductors 1 are covered with a powder-coated insulation2 over which has been applied a liquid resin 3 which has been B-stagedwith ultraviolet light at 4. At 5, the B-staged resin on adjacentstrands of the conductor has been C-staged forming a solid mass 6.

The conductor used in this invention may be of any material, though itis typically a metal such as copper or aluminum. The conductor may beround or rectangular wire or strip.

An insulating coating is applied over the conductor. The coating can beof almost any resin including epoxies, polyamides, polysulfones,polyester amides, and other resins. The coating is preferably an epoxybecause those resins have the best combination of electrical, chemical,and mechanical properties for use in transformers. (See, for example,U.S. Pat. Nos. 4,088,809 and 4,241,101.) The coating must be applied bya powder coating technique such as a fluidized bed, an electrostaticfluidized bed, or an electrostatic spray gun. After the coating has beenapplied to the conductor, it is fully cured. The coating may be of anythickness but it is typically about 3 to about 8 mils. A description ofa suitable powder coating process can be found in U.S. Pat. No.4,127,695, herein incorporated by reference. Since the powder coating istypically cured by heat, it may be desirable to cool the coating inwater or air prior to coating with the UV adhesive. Cooling may bedesirable if the wire is to be wound on a spool before the UV adhesiveis applied, but if the UV adhesive is to be applied immediately, it maybe desirable to have the powder coated wire warm so as to aid in theflow of the viscous UV adhesive. However, the powder coated wire shouldnot be so hot that it cures the UV adhesive to the C-stage. It is, ofcourse, preferable to coat the powder coated wire with the UV adhesiveimmediately in order to avoid the extra steps of winding and unwindingthe conductor.

The adhesive overcoat may be of any liquid resin which can be B-stagedwith ultraviolet light and thermally cured to the C-stage. This can beaccomplished with an ultraviolet curable resin by only partially curingit to the B-stage and then completing the cure to the C-stage usingheat. However, it is more desirable to use a specially prepared resinwhich has two components--a UV curable component and a heat-sensitivecomponent. A two-component resin is easier to work with because theultraviolet light can only cure it to the B-stage and thus it is notnecessary to carefully control the exposure to ultraviolet light as itwould be if the ultraviolet light could cure the resin all the way tothe C-stage. An example of a two-component resin is given in Example 1,Composition A. Another suitable ultraviolet curable adhesive isdescribed by F. A. Sattler in U.S. Pat. No. 4,317,858.

The adhesive overcoat must be a liquid, and it is preferably 100% solidsto reduce air pollution and the cost of recovering solvents. Suitableresins include acrylated epoxies, cationic photo-initiated epoxies,thiol-polyene systems, and acrylo-urethanes. Acrylated epoxies arepreferred as they have the best properties. A resin can be applied byreverse roller coating, by dipping and passing through a die or a wiperof leather or felt, or other technique. Reverse roller coating ispreferred as it produces a thinner and more easily controlled coating.

After the adhesive overcoat has been applied, it is cured to theB-stage. The B-stage is the point at which the coating becomes dry, tackfree, and nonblocking. This occurs when the resin is cured past itsgelation point. The cure to the B-stage is accomplished usingultraviolet light of a frequency and intensity which depend upon theparticular composition used and the speed with which the conductorpasses under the light. After the adhesive overcoat has been cured tothe B-stage, the conductor can be wound onto reels or it can be usedimmediately. The B-staged coating can be of any thickness, but it ispreferably about 0.25 to about 11/2 mils as a thinner coating has a poorbond strength and a thicker coating uses too much space.

In the next step of this invention strands of the conductor are placedside-by-side. The conductors with the adhesive overcoat may be used toform transformer coils, motor coils, transposed cables, or otherstructures where the fusion of adjacent conductors into a solid masswould be desirable.

In the final step of this invention the adhesive overcoat is heated tocomplete its cure to the C-stage. The temperature and time required tocomplete the cure will depend upon the particular adhesive overcoatcomposition that is used.

The following examples further illustrate this invention.

EXAMPLE 1

A 0.114×0.289 inch rectangular aluminum wire was powder coated with anepoxy powder coating resin described in the example of U.S. Pat. No.4,241,101 or 4,088,809, herein incorporated by reference. The powdercoating was cured in a wire tower at a speed of 10-50 ft/min and a towertemperature of 300°-450° C. and had a thickness of 3 to 8 mils. Afterthe powder coated wire had been fused and cured, short lengths of thepowder coated wire were cut and an adhesive overcoat was brushed ontothe wire by hand and cured to the B-stage under ultraviolet radiation.The following ultraviolet curable overcoats were used.

    ______________________________________                                                              Composition                                                                   (parts by weight)                                       Ingredients             A      B      C                                       ______________________________________                                        Solid diglycidyl ether of bisphenol A                                                                 55.3   56.4   55.7                                    sold by Dow Chemical Co. as "DER 662"                                         Tetraethylene glycol diacrylate                                                                       33.0   33.7   33.6                                    Triethanolamine borate in phenoxyethyl                                                                8.3    8.4    8.2                                     acrylate sold by Westinghouse as "WT-17"                                      Isopropyl benzoin ether sold by                                                                       1.3    1.4    --                                      Stouffer as "V-10" photoinitiator                                             Isobutyl benzoin ether sold by                                                                        --     --     1.4                                     Stouffer as "V-30" photoinitiator                                             Fluorinated alkyl ester sold by 3M                                                                    2.1    --     1.1                                     as "FC-430" Surfactant                                                        Picric acid             --     --     0.01                                    ______________________________________                                    

Three pieces of the adhesive coated wire were clamped togetheroverlapping about 1/4 inch and were heated to 130° for six hours ineither air or in kerosene. After cooling the bonded samples weresubjected to tensile shear testing (double lap shear testing) attemperatures from 25° to 175° C. The results are given in the followingtable.

    ______________________________________                                               Overcoat                                                               Test     Composition A                                                                             Composition B                                                                             Composition C                                Temp. (°C.)                                                                     Bld. 3.0-4.0                                                                              Bld. 3.0-4.0                                                                              Bld. 2.5-4.0                                 ______________________________________                                        Cured in                                                                      Kerosene                                                                       25                              2663                                         Cured in Air                                                                   25      1276        1757        2000, 2708                                   100                              2486                                         125                              2351                                         150                              1537                                         175                               638                                         ______________________________________                                    

This example shows that UV sensitive adhesives can be formulated andapplied to powder coated conductors with good tensile shear strengths attemperatures as high as 175° C. It also shows that bonding in kerosenedoes not adversely affect the bond strength of these adhesives.

EXAMPLE II

Rectangular aluminum wire (0.114×0.289 in) was coated with the powderdisclosed in U.S. Pat. No. 4,241,101 in a wire tower, then cured andspooled. Short lengths (˜12 in) were cut and straightened, then coatedmanually with two different UV sensitive adhesives.

    ______________________________________                                        Composition A                                                                 "DER 662" epoxy resin    47.1   pbw                                           Limonene dioxide         31.3   pbw                                           Methyl tetrahydrophthalic anhydride                                                                    15.7   pbw                                           Aliphatic triglycidyl ether sold by                                                                    2.4    pbw                                           Celanese as "5044" epoxy resin                                                triaryl sulfonium hexafluoro phosphate                                                                 5.1    pbw                                           sold by 3M as "FC-508" photoinitiator                                         Chromium acetylacetonate 0.04   pbw                                           Composition B                                                                 "DER 662" epoxy resin    45.5   pbw                                           1,6-hexanediol diacrylate                                                                              6.9    pbw                                           2-ethoxyethyl acrylate   9.2    pbw                                           Butyl glycidyl ether sold by Ciba                                                                      5.0    pbw                                           Geigy as "RD-1" diluent                                                       Diglycidyl ether of neopentyl glycol                                                                   5.0    pbw                                           Methyl tetrahydrophthalic anhydride                                                                    15.0   pbw                                           "V-30" photoinitiator    0.64   pbw                                           Chromium acetylacetonate 0.04   pbw                                           ______________________________________                                    

Wires overcoated with the above composition and B-staged were overlappedin pairs by a distance of 1 in. along their long axes and subjected to apressure of 50 psi. The pairs were placed in a laboratory aircirculating oven for 6 hours at 130° C. to C-stage the adhesiveovercoats. After cooling, the bond pairs were tested for lap shearstrength at 150° C. The results were as follows (average of 5 samples):

    ______________________________________                                        Overcoat      Lap Shear Strength (psi)                                        ______________________________________                                        Composition A  58                                                             Composition B 154                                                             ______________________________________                                    

After the adhesive had been B-staged, some samples were shelf aged for aperiod of 3 months.

The tensile shear test as described in Example I was repeated. Theresults were as follows (average of 5 samples):

    ______________________________________                                        Overcoat      Lap Shear Strength (psi)                                        ______________________________________                                        Composition A  51                                                             Composition B 150                                                             ______________________________________                                    

These results show that the UV adhesives of this invention can beapplied to powder coated conductors and can retain their tensile shearstrength (single lap shear test) after shelf aging for periods of atleast 3 months.

EXAMPLE III

Samples of 0.064×0.258 inch copper wire were coated with 4 mils of epoxypowder coating as in Example I. The samples were then coated withvarious adhesive overcoats including the same epoxy powder coating,Formvar and a UV composition which consisted of

Acrylated epoxy--55.5% (50% phenoxyethyl acrylate);

UV catalyst--2.5%;

Hexamethoxymethyl melamine (sold by American Cyanamid as "Cymel303")--6.4%;

Phenoxyethyl acrylate--6.4%;

Vinyl acetate--8.4%;

Epoxy novolac--6.4%;

"WT-17"--6.0%;

Benzodimethyl melamines--0.18% and blocked acrylated urethane--6.2%;

Tetraethylene glycol diacrylate--1.8%;

Catalyst 10-10 (manufactured by American Cyanamid)--0.03%;

Iron or chromium acetylacetonates--0.03%

The ultraviolet adhesive overcoat was prepared in the following manner:

Three six inch samples were overlapped one inch and clamped togetherunder a pressure of 10 psi. Beam tests were also performed on thesamples. In a beam test, two beams 5 inches apart are placed under astack of 5 wires bonded together and a third beam is pressed downwardbetween the other two beams. The psi required to produce a failure aremeasured. The following table shows the results.

    ______________________________________                                                          Test Temperature                                            Adhesive            R. T.  120° C.                                     ______________________________________                                        Powder Coated Epoxy 480    650                                                Formvar*            830    430                                                UV                  872    736                                                ______________________________________                                         *a wire enamel which contains polyvinyl formyl as a base resin. Other         resins such as phenols, blocked isocyanates, and melamine formaldehyde ar     added, depending on the supplier.                                        

These results show that correctly formulated UV adhesives have beamshear strengths comparable to those of either powdered or solvent basedadhesives when applied over powder coated conductors.

EXAMPLE V

A further benefit of using UV sensitive overcoats is a markedimprovement in electric strength. A spool of 0.064×0.258 copperrectangular wire was coated with powder manufactured by HYSOL and finelyground.

Example I was repeated and the dielectric strength of samples with andwithout UV overcoat were tested. The following table gives the resultson 0.064 by 0.258 inch rectangular copper wire.

    __________________________________________________________________________                     Dielectric (K. Volts)                                                         1st sample  2nd sample                                       Undercoat Overcoat                                                                             #1 #2 #3 #4 #5 #6 #7 #8 Build (Mils)                         __________________________________________________________________________    A diglycidyl ether                                                                      None   4.2                                                                              3.5                                                                              4.1                                                                              2.2                                                                              3.8                                                                              3.8                                                                              4.6                                                                              3.4                                                                              Side 1 - 4.5 Powder Thickness        of bisphenol A epoxy                                                                           avg.                                                                             3.5                                                                              Kv, avg. Kv/mil = 0.82                                                                          Side 2 - 4.0 Powder Thickness        power cured with                                                              trimellitic anhydride                                                                   UV Adhesive                                                                          4.8                                                                              4.5                                                                              4.9                                                                              4.2                                                                              3.7                                                                              4.2                                                                              4.3                                                                              4.5                                                                              Side 1 - 4.0 Total Thickness         sold by 3M Company                                                                             avg.                                                                             4.4                                                                              Kv, avg. Kv/mil = 1.11                                                                          Side 2 - 3.9 Total Thickness         Diglycidyl ether                                                                        None   2.8                                                                              0.3                                                                              2.0                                                                              4.8                                                                              3.9                                                                              0.6                                                                              4.7                                                                              3.9                                                                              Side 1 - 3.0                         of bisphenol A   avg.                                                                             3.0                                                                              Kv, avg. Kv/mil = 0.89                                                                           Side 2 - 3.7                        epoxy power                                                                   (See U.S. Pat. No.                                                                      UV Adhesive                                                                          4.5                                                                              4.5                                                                              5.0                                                                              5.4                                                                              4.9                                                                              5.4                                                                              5.0                                                                              3.6                                                                              Side 1 - 3.5                         4,241,101)       avg.                                                                             4.9                                                                              Kv, avg. Kv/mil = 1.30                                                                          Side 2 - 4.1                         __________________________________________________________________________     This experiment shows that the addition of a UVsensitive adhesive overcoa     increases the electric strength in Kv/mil of a powder coated conductor by     at least 20%. This is believed to be due to the initially liquid UV           sensitive filling any voids or thinner areas in the powder coating.      

EXAMPLE VI

Three samples of rectangular aluminum wire coated with the same powderused in Example I were dipped into a UV sensitive resin comprised of:

    ______________________________________                                        "DER 662" epoxy resin   502.5  g                                              "WT-17"                 75.0                                                  Tetraethylene glycol diacrylate                                                                       200.0                                                 Ethyl hexyl acrylate    150.0                                                 2-hydroxy ethyl acrylate                                                                              37.5                                                  "V-10" photoinitiator   3.75                                                  Tert-butyl perbenzoate  3.75                                                  ______________________________________                                    

Excess resin was wiped off and the coating was irradiated for 8 minutesunder a source of ultraviolet light.

After the irradiation, the coating was dry to the touch and measured 3.5mil (addition to the thickness).

The three samples were pressed together under nominal spring pressure(from a bulldog clip) at 150° C. for 11/2 hours.

A double lap-shear test gave a test value of 184 lbs. on the two adheredareas of 0.350×0.258 in., equivalent to 1020 psi.

We claim:
 1. An article comprising a conductor coated with a fused andcured resin powder applied insulating coating, over which is a coatingof an ultraviolet B-stageable, thermally C-stageable liquid resin.
 2. Anarticle according to claim 1 wherein said conductor is an aluminum orcopper wire.
 3. An article according to claim 1 wherein said insulatingcoating is an epoxy resin.
 4. An article according to claim 1 whereinsaid insulating coating is about 3 to about 8 mils thick.
 5. An articleaccording to claim 1 wherein said liquid resin is an acrylated epoxy. 6.An article according to claim 1 wherein said liquid resin issolventless.
 7. An article according to claim 1 wherein said coating hasbeen cured to the B-stage with ultraviolet light.
 8. An articleaccording to claim 7 wherein strands of said conductor are placedside-by-side and said coating is cured to the C-stage.
 9. An articleaccording to claim 8 in the form of a transformer coil.